1. Field of the Invention
The embodiments of the present invention relate to an image sensor, and more particularly, to a CMOS image sensor having an optical block area set in a pixel array.
2. Description of the Related Art
With the development of electronic communication, digital devices having a digital processing function, such as digital cameras, mobile phones, game machines, and micro cameras, have been rapidly spread. Most of the digital devices include an image sensor required for taking an image.
The image sensor senses an image. Specifically, the image sensor converts an image inputted as light from outside into an electrical signal and transmits the electrical signal to a digital processing device. Examples of the image sensor may include a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
The CCD image sensor includes a photodiode, a CCD, and a signal detection circuit, which are formed over a P-type impurity layer. The photodiode serves to convert light incident from outside into an electric charge, the CCD serves to transmit the electric charge to the signal detection circuit, and the signal detection circuit serves to convert the electric charge into a voltage.
The CMOS image sensor includes CMOS transistors each configured in a complementary manner to join a PMOS (P channel Metal Oxide Semiconductor) transistor and an NMOS (N channel Metal Oxide Semiconductor) transistor, in order to convert an input image into an electrical signal. The CMOS image sensor has an advantage in which it has high integration degree and low power consumption.
The image sensor includes a light sensing unit configured to receive light incident from outside and generate and store photo-charges. The image sensor further includes a color filter disposed over the light sensing unit. The color filter may have three colors of red, green, and blue or three colors of yellow, magenta, and cyan.
In particular, the CMOS image sensor includes an optical block area for blocking light from being irradiated onto pixels. The pixels formed in the optical block area are optional pixels for black level correction, and are used for controlling a pixel output. Since the optical block area uses a pixel output as an offset value in a dark state, the optical block area is generally implemented in a state in which the introduction of light is blocked. As such, the pixels in the optical block area determine the offset value in place of main pixels. Thus, when an asymmetry occurs between the main pixels and the pixels in the optical block area, the noise characteristic of the image sensor may be seriously degraded.
Furthermore, the pixels in the optical block area receive plasma damage after a back-end process, or particularly, after an etching process. In order to cure the plasma damage, UV erase, that is, UV anneal may be performed. At this time, the pixels in the optical block area have a UV curing effect different from the main pixels, because UV light is blocked during the curing process. Thus, the dark current characteristic of the pixels in the optical block area may differ, and a serious asymmetry may occur between the pixels. As a result, the noise characteristic of the CMOS image sensor may be degraded.